1. Field of the Invention
The present invention relates to a process for hardcoating polycarbonate sheet and, more particularly, relates to a process for providing a high quality, abrasion and chemical resistant coating onto a thick, rigid polycarbonate substrate.
2. Description of Related Art
The substitution of glass with sheets made from aromatic polycarbonates provides the major advantage of shatter resistance and weight reduction but comes with the drawbacks of low mar and scratch resistance. Attempts to improve the abrasion resistance of these polycarbonate sheets have included the application of various coatings, such as silicone coatings, solvent-free acrylic coatings and solvent-based acrylic coatings, to the surface thereof. Silicone coatings, however, typically have a number of shortcomings including the use of solvents therein to decrease viscosity and enhance leveling during application thereof. These solvents are expensive, pose potential environmental hazards and require substantial periods of time and energy to effect removal thereof during drying of the coatings. Silicone coatings also suffer from the requirement of substantial heat application and substantial cure times to effect curing and hardening of the coating layer. These drawbacks result in a somewhat slow and expensive coating process.
Solvent-free acrylic coatings have also had a number of problems associated with their application to aromatic polycarbonate sheets. Namely, solvent-free acrylic coatings have relatively high viscosities often in excess of 50 centipoise and sometimes as high as 5000 centipoise resulting in their tendency to resist leveling after application to the sheet thus resulting in an unacceptable degree of imperfections such as roughness, waviness and striations in the final hardcoat. For example, roll coating of solvent-free acrylic coatings onto polycarbonate substrates often results in the presence of visible striations in the final hardcoat. Spraying techniques for application of these high viscosity coatings result in bubbles which are visible in the cured coating, and the employ of processes such as dip coating are typically very slow production processes which do not permit high-speed on-line production. An additional problem associated with acrylic coatings is that when they are cured by ultraviolet radiation, they must be in a substantially diatomic oxygen-free atmosphere, and thus prior acrylic sheet coating processes have typically required the use of a nitrogen purge chamber for the ultraviolet radiation curing thereof. The nitrogen purge chambers add equipment requirements and potential suffocation hazards to the process. Prior solutions to overcome the problems associated with the use of high viscosity acrylic coatings on polycarbonate sheets have included the addition of solvents thereto to reduce the viscosity of the coatings and promote their leveling tendencies. Solving the problem of high viscosity by the addition of solvents, however, simply trades off one problem for another. Specifically, while the problem of high viscosity may have been solved, the aforementioned problems associated with solvents have been added to the system.
Additional problems have also been associated with prior processes for the production of hardcoated polycarbonate sheet. Specifically, when polycarbonate sheet is extruded it must be trimmed at its outer edges to form substantially rectangular sheets of uniform width and length. The strips of polycarbonate trimmed from the sheets are then either discarded, which is wasteful and which adds cost to the process, or the scrap is recycled into the resin of subsequent sheets. Additionally, poor quality hardcoated sheets resulting from hardcoating operations can either be discarded, which is wasteful and costly, or may be chopped up and recycled into resin for subsequent sheet production. As a practical matter, however, whenever recycled polycarbonate plastic is employed in the production of polycarbonate sheets, the resultant sheets often have surface defects and surface characteristics that are undesirable. For example, small indentations may be present at the surface or small pieces of dirt or sand may be present at the surface. These surface defects are typically still visible after the application of thin acrylic coatings thereto by conventional sheet coating techniques and the employ of thick solvent-free acrylic coatings is generally too expensive and too difficult to effectively cure. Thus, it has been generally impractical in the past to produce high surface quality coated sheet which employs recycled or low quality polycarbonate resin.
Accordingly, one object of the present invention is to provide an improved process for applying solvent-free acrylic hardcoats to polycarbonate sheet.
Another object is to provide a coating process which will effectively mask surface imperfections in the underlying sheet.
A further object is to provide a high-speed process for producing high quality coatings on polycarbonate sheet.